The impact absorbers used for shoes and such are required to meet all of such property requirements as high mechanical strength, resistance to hardening in the cold districts (low-temperature hardening resistance), low specific gravity and low impact resilience (impact absorption properties).
In recent years, resins such as polystyrene, polyethylene and ethylene-vinyl acetate copolymer, natural rubber, and elastomers such as styrene-butadiene copolymer and acrylonitrile-butadiene copolymer have been used as base polymer for the impact absorbers, but these conventional impact absorbers are not necessarily satisfactory in performance in practical use, and a variety of methods for improving their requisite properties have been proposed.
More specifically, Patent Document 1 proposes a copolymer comprising a conjugated diene and an aromatic vinyl monomer and having a tan δ peak temperature in the range of 20° C. to 40° C. as a polymer for the impact absorbing foams. In this patent document, however, no disclosure is made other than the tan δ temperature. In the Examples of Patent Document 1, there are merely disclosed the hydrogenated copolymers having a tan δ peak temperature of 0° C.
Patent Document 2 proposes a copolymer comprising a conjugated diene and an aromatic vinyl monomer and having a tan δ peak temperature of −20° C. or higher and a peak value of tan δ of 0.3 or more as a polymer for the impact absorbing foams. This document, however, discloses nothing other than the tan δ peak temperature and merely shows the hydrogenated copolymers having a tan δ peak temperature of 27° C. in the Examples.
Patent Document 3 proposes combination use of a copolymer comprising a conjugated diene and an aromatic vinyl monomer and having a tan δ peak at a temperature in the range of −20° C. to 40° C., and a copolymer comprising a conjugated diene and an aromatic vinyl monomer and having a tan δ peak at a temperature in the range of −70° C. to −20° C. as a polymer for the impact absorbing foams. In this document, however, no disclosure other than the tan δ peak temperatures is made. In the Examples of Patent Document 3, there are proposed the copolymers whose tan δ peak temperature measured at 110 Hz is 5° C. (The tan δ peak temperature at 1 Hz as calculated from the ordinary temperature-time conversion described in, for instance, Non-patent Document 1 is 0° or below).
Patent Document 4 proposes the copolymers which are controlled in distribution of aromatic vinyl monomer and whose tan δ peak temperature is specified to be from −60° C. to 0° C. as a preferred range. In this document, however, no disclosure other than the peak temperature is made.    Patent Document 1: JP-A-05-345833    Patent Document 2: JP-A-2003-128870    Patent Document 3: JP-A-2004-107519    Non-patent Document 1: Rheology Data Handbook, Maruzen, 2006.    Patent Document 4: US 2003/0181584